See Figures 1, 2, 3 and 4
The points function as a circuit breaker for the primary circuit of the ignition system. The ignition coil must boost the 12 volts of electrical pressure supplied by the battery to as much as 25,000 volts in order to fire the plugs. To do this, the coil depends on the points and the condenser to make a clean break in the primary circuit.
The coil has both primary and secondary circuits. When the ignition is turned ON, the battery supplies voltage through the coil and onto the points. The points are connected to ground, completing the primary circuit. As the current passes through the coil, a magnetic field is created in the iron center core of the coil. As the cam in the distributor turns, the points open and the primary circuit collapses. The magnetic field in the primary circuit of the coil also collapses and cuts through the secondary circuit windings around the iron core. Because of the scientific phenomenon called electromagnetic induction, the battery voltage is increased to a level sufficient to fire the spark plugs.
When the points open, the electrical charge in the primary circuit jumps the gap created between the two open contacts of the points. If this electrical charge were not transferred elsewhere, the metal contacts of the points would melt and the gap between the points would start to change rapidly. If this gap is not maintained, the points will not break the primary circuit. If the primary circuit is not broken, the secondary circuit will not have enough voltage to fire the spark plugs.
The function of the condenser is to absorb excessive voltage from the points when they open and thus prevent the points from becoming pitted or burned.
It is interesting to note that the above cycle must be completed by the ignition system every time a spark plug fires. In a V8 engine, all of the spark plugs fire once for every two revolutions of the crankshaft. That means that in one revolution, 4 spark plugs fire. So, when the engine is at an idle speed of 800 rpm, the points are opening and closing 3,200 times a minute.
There are two ways to check the breaker point gap. It can be done with a feeler gauge or a dwell meter. Either way you set the points, you are basically adjusting the amount of time that the points remain open. The time is measured in degrees of distributor rotation. When you measure the gap between the breaker points with a feeler gauge, you are setting the maximum amount the points will open when the rubbing block on the points is on a high point of the distributor cam. When you adjust the points with a dwell meter, you are adjusting the number of degrees that the points will remain closed before they start to open as a high point of the distributor cam approaches the rubbing block of the points.
When you replace a set of points, always replace the condenser at the same time.
When you change the point gap or dwell, you will also have changed the ignition timing. So, if the point gap or dwell is changed, the ignition timing must be adjusted also.
INSPECTION OF THE POINTS
- Disconnect the high tension wire from the top of the distributor and the coil.
- Remove the distributor cap by prying off the spring clips on the sides of the cap or by turning the screwheaded fasteners.
- Remove the rotor from the distributor shaft by pulling it straight up. Examine the condition of the rotor. If it is cracked or the metal tip is excessively worn or burned, it should be replaced.
- Pry open the contacts of the points with a screwdriver and check the condition of the contacts. If they are excessively worn, burned or pitted, they should be replaced.
- If the points are in good condition, adjust them and replace the rotor and the distributor cap. If the points need to be replaced, follow the replacement procedure given below.
See Figures 5 through 12
- Remove the coil high tension wire from top of the distributor cap. Remove the distributor cap from the distributor and place it out of the way. Remove the rotor from the distributor shaft.
- Loosen the screw that holds the condenser lead to the body of the breaker points and remove the condenser lead from the points.
- Remove the screw that holds and grounds the condenser to the distributor body. Remove the condenser from the distributor and discard it.
- Remove the points assembly attaching screws and adjustment lockscrews. A screwdriver with a holding mechanism will come in handy here, so that you don't drop a screw into the distributor and have to remove the entire distributor to retrieve it.
- Remove the points by lifting them straight up and off the locating dowel on the plate. Wipe off the cam and apply new cam lubricant. Discard the old set of points.
- Slip the new set of points onto the locating dowel and install the screws that hold the assembly onto the plate. Do not tighten them all the way.
- Attach the new condenser to the plate with the ground screw.
- Attach the condenser lead to the points at the proper place.
- Apply a small amount of cam lubricant to the shaft where the rubbing block of the points touches.
ADJUSTMENT WITH A FEELER GAUGE
See Figures 13 and 14
- If the contact points of the assembly are not parallel, bend the stationary contact so that they make contact across the entire surface of the contacts. Bend only the stationary bracket part of the point assembly, not the moveable contact.
- Turn the engine until the rubbing block of the points is on one of the high points of the distributor cam. You can do this by either turning the ignition switch to the start position and releasing it quickly (bumping the engine) or by using a wrench on the bolt that holds the crankshaft pulley to the crankshaft.
- Place the correct size feeler gauge between the contacts. Make sure it is parallel with the contact surfaces.
- With your free hand, insert a screwdriver into the notch provided for adjustment or into the eccentric adjusting screw, then twist the screwdriver to either increase or decrease the gap to the proper setting.
Delco V6 and V8 distributors have point sets which are adjustable with an allen wrench.
- Tighten the adjustment lockscrew and recheck the contact gap to make sure that it didn't change when the lockscrew was tightened.
- Replace the rotor and distributor cap, and the high tension wire that connects the top of the distributor and the coil. Make sure that the rotor is firmly seated all the way onto the distributor shaft and that the tab of the rotor is aligned with notch in the shaft. Align the tab in the base of the distributor cap with the notch in the distributor body. Make sure that the cap is firmly seated on the distributor and that the retainer springs or L-shaped, screwheaded clips are in place. Make sure that the end of the high tension wire is firmly placed in the top of the distributor and the coil.
ADJUSTMENT WITH A DWELL METER
- Adjust the points with a feeler gauge as previously described.
- Connect the dwell meter to the ignition circuit according to the manufacturer's instructions.
- If the dwell meter has a set line on it, adjust the meter to zero the indicator.
- Start the engine.
Be careful when working on any vehicle while the engine is running. Make sure that the transmission is in Neutral and that the parking brake is applied. Keep hands, clothing, tools and the wires of the test instruments clear of the rotating fan blades.
- Observe the reading on the dwell meter. If the reading is within the specified range, turn off the engine and remove the dwell meter.
If the meter does not have a scale for 4-cylinder engines, multiply the 8-cylinder reading by two.
- If the reading is above the specified range, the breaker point gap is too small. If the reading is below the specified range, the gap is too large. In either case, the engine must be stopped and the gap adjusted in the manner previously covered.
On the 6-225 engine and all of the V8 engines, it is possible to adjust the dwell while the engine is running. There is a little window in the side of the distributor that can be raised, so that the points can be adjusted with an allen wrench.
After making the adjustment, start the engine and check the reading on the dwell meter. When the correct reading is obtained, disconnect the dwell meter.
- Check the adjustment of the ignition timing.